This article presents an experimental study by holographic interferometry of the following material properties of conventional and negative Poisson's ratio copper foams: Young's moduli, Poisson's ratios, yield strengths, and characteristic lengths associated with inhomogeneous deformation. The Young's modulus and yield strength of the conventional copper foam were comparable to those predicted by microstructural modelling on the basis of cellular rib bending. The re-entrant copper foam exhibited a negative Poisson's ratio as indicated by the elliptic contour fringes on the specimen surface in the bending tests. Inhomogeneous, non-affine deformation was observed holographically in both foam materials. Download pdf

The indentation resistance of foams, both of conventional structure and of a novel re-entrant structure giving rise to negative Poisson's ratio, was studied using holographic interferometry. In holographic indentation tests, re-entrant foams had higher yield strengths sigma y and lower stiffness E than conventional foams of the same original relative density. Damage in both kinds of foam occurred primarily directly under the indenter. Calculated energy absorption for dynamic impact is considerably higher for re-entrant foam than conventional foam.

Holographic methods are utilized to examine deviations from classical elasticity in a cellular solid, polymethacrylamide closed cell foam. A square cross section bar is subjected to static torsional deformation. The warp deformation is observed to be less in a foam bar than in a homogeneous polymeric bar used as a control. The homogeneous bar obeys the predictions of classical elasticity. Behavior of the foam bar is consistent with Cosserat elasticity. In a Cosserat solid, points in the continuum to rotate as well as translate, and the material supports couple per unit area as well as force per unit area. Cosserat effects can lead to enhanced toughness. This image shows holographic fringes associated with warp.

Development of holographic methods

Lakes, R. S., "Multi wavelength techniques in holographic interferometry", Journal of Modern Optics, 35(9), 1459-1465 (1988).
Techniques are presented which take advantage of the wavelength dependence of various phenomena in holographic interferometry. Image-plane interferograms illuminated with light containing multiple wavelengths exhibit color dispersion of the fringes. We extract from this dispersion, full- field information concerning displacement components which are not disclosed by monochromatic illumination.

A simple correction scheme is presented, which permits the use of large aperture lens systems of modest quality, even single element lenses, to produce image plane holograms viewable in white light. The present method allows white light reconstruction and corrects field curvature, which is the most objectionable aberration in display holograms. The field curvature was corrected by making the hologram with diverging light and illuminating the hologram with collimated light to introduce a compensating negative curvature of field.

A method is presented which permits control of the reconstruction wavelength of reflection holograms and holographic optical elements [HOE's]. This approach makes use of developer and bleach which minimize emulsion shrinkage combined with control of ambient humidity to control the emulsion shrinkage during formation and reconstruction. A simple index matching approach to the elimination of the wood grain effect in reflection holograms is also presented.

In some applications of diffusers, it is desirable to minimize the diffuse back reflection of light. Use of polarized light is one way to reduce this back reflection. To that end, the effect of diffusers upon polarized light is studied experimentally. Diffusers based on ground glass, white plastic containing scatterers, and holographic optical elements are considered. The ground glass and HOE diffusers preserve polarization in the diffusion process, but the white plastic does not. Diffuse back reflection from ground glass or holographic diffusers can be significantly reduced by the use of an isolator based on a quarter wave plate.

Holographic diffusers were prepared using silver halide (Agfa 8E75 and Kodak 649F) and photopolymer (Polaroid DMP128 and DuPont 600, 705, and 150 series) media. It was possible to control the diffusion angle in three ways: by selection of the properties of the source diffuser, by control of its subtended angle, and by selection of the holographic medium. Several conventional diffusers based on refraction or scattering of light were examined for comparison.

We present a holographic element capable of projecting dynamic stereo images, and allowing the observer to see through the device, for possible use as a head up display in aircraft. The device is based on a volume reflection holographic optical element which contains two sets of Bragg planes. Each set of Bragg planes diffracts light from a two-dimensional source to the appropriate eye to achieve a stereo effect.
Lakes, R. S. and Vick, G., "Partial collimation of light from a diffusely reflective source", J. Modern Optics, 39, 2113- 2119, (1992).

A general purpose collimator capable of collimation of radiation from an arbitrary thermal source of diffuse light is incompatible with the second law of thermodynamics. However there are 'special purpose' collimators which would not be generally applicable. A new collimator which is effective when placed close to a white (diffusely reflective) source is presented.

Holographic diffusers have been produced with very low back diffusion in comparison with diffusion in the forward direction. Reduced back diffusion was achieved by lamination and index matching procedures which minimized the formation of Bragg planes parallel to the film surface. Photopolymer media were used as phase media. Diffusers with the lowest values of back diffusion were prepared by moderately restricting the field angle of incident light during formation.